The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100\% if phosphorescent emitters to harvest triplet excitons are used; however, the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are, for example, waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. Additionally, absorption in the organic layers and the electrodes can play a role. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing simulation parameters, for example, the distance of the emitter material to the cathode or thicknesses of the various layers, enables us to study their influence on the fraction of light leaving the OLED. An important parameter in these simulations and for the actual device is the radiative quantum efficiency q, which is defined as the efficiency of radiative exciton decay in an unbounded space filled by the emitting dye and its matrix. The simulations show that due to microcavity effects the radiative decay channel can be considerably changed in an OLED as compared to free space emission of a dipole. Thus the knowledge of the radiative quantum efficiency is crucial for the optimization of OLEDs. As an example, we present simulations of bottom-emitting OLEDs based on the well-known green emitter tris-(8-hydroxyquinoline) aluminum with transparent indium tin oxide anode and a calcium/aluminum cathode.